Composite materials formed from a matrix of plastic material charged with microscopic particles such as glass fibers are already employed for the manufacture of elements of watch cases, in particular monolithic cases and case bands. Such elements are manufactured by injection which gives the advantage of greatly reducing the costs of manufacture. Plastic materials enable the realization of watch cases which are light and of various colours.
These two factors have been determinative in the success encountered by such products. At the same time this success is limited exclusively to the lowest price watches. This is essentially due to the fact that the very low hardness of the plastic renders it vulnerable to the numerous attacks to which most wrist watch cases are daily subjected owing to the various activities professional or leisure practised by the wearers of such watches.
It has already been proposed to obtain cases by the sintering of hard materials such as carbides or nitrides. At the same time, the production of such cases is limited to a higher price category of watches because of the cost of manufacture thereof.
It has also been proposed to obtain the exterior parts of the watch by use of natural materials such as mineral or rock materials or even sea shells. Such a type of manufacture is described in the French patent document FR Pat. No. 2,178,032 and in the corresponding U.S. Pat. No. 3,861,990. In both of these documents the pieces are obtained by sintering and do not in any case concern a composite material in the sense of the term used herein. In the French document however the next to last paragraph of the description states that although one has described only a single hot sintering procedure following compression, one may obtain results similar thereto by means of a binder in a procedure where an adhesive is employed or injection moulding in combination with plastic materials. This statement however is not supported by any example which would set forth at least one manufacturing method proposing a composite material where the mineral particles occupy 60 to 95% of the said material as is the case in the present invention. In effect at the filing date of the cited documents there was known only a mixture of fibers or glass beads with the plastic material in a proportion not exceeding 25%. Moreover, the same document foresees only the utilization of powder thus of microscopic particles which can lead to easily scratchable visible surfaces.
For a watch case, the mechanical properties required are in fact critical only at the surface and especially on the exposed surfaces subject to attacks due to shocks and scratches. The inclusion of microscopic particles has as effect the modification of the elastic properties of the plastic, above all vis-a-vis the stresses such as shear, compression, tension, bending etc. On the other hand, such charges have a less pronounced effect vis-a-vis the surface hardness because of their microscopic dimensions. In effect a cutting element can scratch the surface of such a reinforced plastic by passing between the particles in view of their dimension. It appears thus that an effective scratch protection of the surface of a part in plastic material does not depend alone on the hardness of the particles used as reinforcement and of the proportion of such particles. Moreover, a too high proportion of microscopic particles may have as a consequence the rendering of the resin less resistant to shocks which is evidently undesirable for a watch case which includes in particular edges capable of being broken off if the material of the case is brittle.
The purpose of this invention is to overcome at least partially the difficulties mentioned hereinabove.